The present invention relates to a slit exposure optical system for electrophotographic copying machines.
As the representative coventional slit exposure systems, there are known two types. In one type, the original is moved while the exposure optical system is set stationary. In the other type, the original is placed in a predetermined position and the exposure optical system is moved.
In the history of improvement of copying machines, increasing the speed of copying has always been one of the guiding principles for improvement of copying machines. From the viewpoint of increasing the speed of copying, the slit exposure system having a movable exposure optical system, while setting the original stationery, is more suitable for increasing the speed of copy than the original-moving type slit exposure system, since in the original-moving type, the inertia of the moving portion itself is so great that it is difficult to reciprocate the moving portion at a high speed. Furthermore, depending upon the kind of originals to be copied, for instance, depending upon whether the original is a sheet or a book, the inertia of the moved portion varies. Therefore, it is difficult to move various originals at a constant speed.
In a typical slit exposure system of the type in which the exposure optical system is moved relative to a stationary original, an image formation lens system is fixed, while a pair of plane mirrors, which are disposed on the side of the original with respect to an exposure light path, are moved at a 2:1 speed ratio, and the original is scanned. In this system, one of the plane mirrors is reciprocated over the full length of the original to be copied in the scanning direction of the original, and its backward movement does not contribute to the copying of the original. Therefore, in order to increase the copying speed, the backward movement speed of the pair of the plane mirrors has to be increased as much as possible. As a matter of course, there is a limit to the increase of the backward movement speed.
In order to overcome this limit, the reduction of the movement path of the exposure optical system is considered. As a matter of fact, a method of performing slit exposure while moving the image formation lens system is proposed, for example, in Japanese laid-open patent application No. 50-87654, or a method of moving both the image formation lens system and the original is proposed, for example, in Japanese patent publication No. 49-31333. The inventor of the present invention proposed a system for moving a pair of plane mirrors, disposed on the side of a photoconductor, relative to an image formation lens system in Japanese laid-open patent application No. 53-102041.
In the above-mentioned methods of moving the image formation lens system or the pair of plane mirrors, the moving distance of the image formation lens system or of the pair of plane mirrors is half of the length of the original in the scanning direction thereof and therefore, the adoption of these slit exposure systems has increased the copying speed. For instance, in the system of the type of moving the image formation lens system, 75 B4-size copies can be obtained per minute. On the other hand, in the system of the type when both the image formation lens system and the original when moved at the same time, the moving distance of the plane mirrors can be reduced further in comparison with the above-mentioned two methods. However, this system is not promising with respect to high speed copying on the ground that transporting original is still unsuitable for high speed copying.